Before starting, confirm that the Emscripten configuration file exists and
specifies all settings correctly. This file is available as ~/.emscripten
on UNIX-like systems and %USERPROFILE%\.emscripten on Windows. It’s usually
written by the Emscripten SDK, e.g. when invoking emsdkactivatelatest,
or by your package manager. It’s also created when starting Emscripten’s
emcc program if the file doesn’t exist.

Attention

On Windows, make sure to escape backslashes of paths within the Emscripten
configuration file as double backslashes \\ or use Unix-style paths with a
single forward slash /.

Open a terminal and navigate to the root directory of the engine source code.
Then instruct SCons to build the JavaScript platform. Specify target as
either release for a release build or release_debug for a debug build:

The engine will now be compiled to WebAssembly by Emscripten. Once finished,
the resulting file will be placed in the bin subdirectory. Its name is
godot.javascript.opt.zip for release or godot.javascript.opt.debug.zip
for debug.

Finally, rename the zip archive to webassembly_release.zip for the
release template:

WebAssembly can be compiled in two ways: The default is to first compile to
asm.js, a highly optimizable subset of JavaScript, using Emscripten’s
fastcomp fork of LLVM. This code is then translated to WebAssembly using a
tool called asm2wasm. Emscripten automatically takes care of both
processes, we simply run SCons.

The other method uses LLVM’s WebAssembly backend. This backend is available
starting with LLVM 8 or in development builds.
Emscripten manages this process as well, so we just invoke SCons.

In order to choose one of the two methods, the LLVM_ROOT variable in the
Emscripten configuration file is used. If it points to a directory containing
binaries of Emscripten’s fastcomp fork of clang, asm2wasm is used.
This is the default in a normal Emscripten installation. Otherwise,
LLVM binaries built with the WebAssembly backend will be expected and
the LLVM’s WebAssembly backend is used.